The biggest problem for me in doing custom symbols is the measurements and the subsequent drawing. My life goal is as yours, manage to create a custom symbol and also build my own custom dev board. Unfortunately many things got in the way and the work kept delaying for almost 9-10 years now.

The reason I picked a particular SD card was because it was low cost. My country has crazy import fees and because of this I am forced to select low cost parts which usually have bad datasheet and documentation.